Internal DNS not passing through for some reason only on RPi

[aram535]

Versions

• Pi-hole: v5.17.1
• AdminLTE: v5.20.1
• FTL: v5.23

Platform

• OS and version: 10 (buster)
• Platform: Raspberry Pi

Expected behavior

PiHole should check the upstream DNS resolution for the internal domain.

Actual behavior / bug

I have an odd issue. I normally have 2 piholes running (primary and secondary).

Both are running the same version:

When I query the primary pihole with external or internal addresses good values return without any issue.
When I query the secondary pihole with external, returns good values return without any issue.
On the secondary ONLY - if I query the internal it sort of hiccups every once in a while and stops responding. All returns are empty (address not found) NXDOMAIN.
The fix is to pihole restartdns it starts to work for a while.

These are from local on the secondary pihole (RPi):
pi@pihole:~ $ dig +short u/localhost -p 53 homeassistant.basement.lab
pi@pihole:~ $ dig +short u/localhost -p 5353 homeassistant.basement.lab
192.168.1.8
pi@pihole:~ $ pihole restartdns
  [✓] Restarting DNS server
pi@pihole:~ $ dig +short u/localhost -p 53 homeassistant.basement.lab
192.168.1.8

I went through side-by-side and compared the configuration of pihole, looks the same. With Bind9 I copied the entire configuration over and just saved master to slave so they're identical.

Steps to reproduce

Steps to reproduce the behavior:

Just wait ... the amount of time does not seem to be consistent. I have setup a cron to check every 3 minutes, if the internal DNS does gets no answer, it does a pihole restartdns

Debug Token

• URL: https://tricorder.pi-hole.net/o7LNvBSo/

Additional context

[-------------] [-------------]
[ primary #53 ] [secondary#53 ]
[ VM - Ubuntu ] [ RPi 3B/1GB  ]
[ 192.168.1.3 ] [ 192.168.1.2 ]
[ ----------- ] [ ----------- ]
[ bind9#5353  ] [ bind9#5353  ]
[ slave zones ] [ master zones]
[ ----------- ] [ ----------- ]

primary: The upstream DNS server is set to Custom1 (IPv4): 192.168.1.3#5353, Customer 2(IPv4): 192.168.1.2#5353
secondary: The upstream DNS server is set to Custom1 (IPv4): 192.168.1.2#5353, Custom2 (IPv4): 192.168.1.3#5353